The present invention relates to a convenient and efficient method for the preparation of an aqueous emulsion of a long-chain alkyl-modified silicone, i.e. organopolysiloxane, having excellent stability against long-term storage, dilution with water and mechanical shearing force.
A long-chain alkyl-modified silicone, which is an organopolysiloxane compound having long-chain alkyl groups substituting for at least apart of the organic groups directly bonded to the silicon atoms in the molecular structure and referred to simply as an alkyl-modified silicone hereinafter, is a known compound and is widely employed as a coating agent on various shaped articles of plastic resins or rubbers and as a metal mold releasing agent for molding of resins and rubbers by virtue of the good coating workability as compared with simple dimethyl silicones. It is usual from the standpoint of safety and working efficiency that such an alkyl-modified silicone is used in the form of a solution prepared by dissolving the silicone in an organic solvent (Japanese Patent Publications 45-10056 and 3-11606) or in the form of an aqueous emulsion. Alternatively, silicones can be used in these applications in the form of an aqueous emulsion prepared by dispersing and emulsifying the silicone in an aqueous medium (Japanese Patent Publication 8-2475 and Japanese Patent Kokai 5-287204).
Alkyl-modified silicones are also used as a surface protecting agent and water-repellent agent by virtue of their excellent slipperiness, water repellency and other surface properties (Japanese Patent Kokai 11-92694, 2000-63755 and 2000-239656).
As a consequence of introduction of long-chain alkyl groups substituting on the silicon atoms, however, alkyl-modified silicones are generally less dispersible and emulsifiable in an aqueous medium than dimethyl silicones so that aqueous emulsions of an alkyl-modifled silicone disadvantageously suffer from poor stability against dilution with water and mechanical shearing. With an object to overcome this disadvantage, Japanese Patent Kokai 5-287204 proposes an emulsion composition compounded with an alkyl diphenyl ether disulfonate. The improvement on the stability of the emulsion composition obtained by this formulation, however, is limited and still far from satisfactory.
When the content of long-chain alkyl groups is increased, alkyl-modified silicones are no longer in the form of a liquid but are converted into a solid. Such a solid silicone must be dispersed and emulsified in an aqueous medium for emulsification after melting by heating or in a hot medium. This means that the emulsifiability of such a solid silicone is still poorer than that of liquid silicones adversely affecting, as a trend, the stability of the aqueous emulsion once prepared.
In addition, it is a usual procedure for the preparation of an aqueous emulsion of an alkyl-modified silicone that synthetic preparation of a silicone substituted by long-chain alkyl groups must precede mechanical dispersion and emulsification of the silicone in an aqueous medium necessitating a complicated two-step procedure. When the alkyl-modified silicone is a solid containing a large amount of long-chain alkyl groups, in particular, the process of emulsion preparation therewith involves a further serious problem in the workers"" safety because the procedure must include handling of a molten silicone at a high temperature as is mentioned above.
The present invention accordingly has an object to provide a novel and efficient method for the preparation of an aqueous emulsion of an alkyl-modified silicone having excellent stability against dilution with water and mechanical shearing without the problem of safety even when the content of the long-chain alkyl groups is high in the alkyl-modified silicone.
The unexpected discovery leading to the present invention obtained by the extensive investigations of the inventors is that the above mentioned object of the invention can be accomplished when the synthetic reaction of an alkyl-modified silicone is conducted in situ in an aqueous emulsion of the starting reactants of the alkyl-modified silicone instead of dispersing and emulsifying a ready-prepared alkyl-modified silicone in an aqueous medium.
Thus, the method of the present invention for the preparation of an aqueous emulsion of an alkyl-modified silicone comprises the steps of:
(a) dispersing and emulsifying an organopolysiloxane having hydrogen atoms directly bonded to the silicon atoms, referred to as an organohydrogenpolysiloxane hereinafter, and an xcex1-(long-chain alkyl) olefin compound, referred to simply as an xcex1-olefin compound hereinafter, represented by the general formula CH2xe2x95x90CHxe2x80x94CpH2p+1, in which the subscript p is a positive integer not smaller than 10, as the starting reactants jointly in an aqueous medium containing a surface active agent to give an aqueous emulsion of the reactants;
(b) adding a catalytically effective amount of a platinum compound to the aqueous emulsion of the reactants; and
(c) heating the aqueous emulsion under agitation to an elevated temperature to effect hydrosilation reaction between the silicon-bonded hydrogen atoms in the organohydrogenpolysiloxane and the ethylenically unsaturatied bonds in the xcex1-olefin compound.